A long recognized problem in welding, specifically electric arc welding, is arc stray or drift. This problem often results in a poor weld due to the varying, nonuniform blow of the arc on the seam to be welded. A variety of methods and apparatus have been devised to control arc stray. U.S. Pat. No. 1,947,077 to Chapman, for example, utilizes an electric current through a backing strip or U-shaped element that creates a magnetic field. This field coacts with the field set up by the welding electrode to "blow" the arc forward in advance of the electrode as it moves along the seam. U.S. Pat. No. 388,245 to Bernardos utilizes an electromagnet located beneath the plates to be welded to influence the electric arc U.S. Pat. No. 2,773,969 to Gunther utilizes a "magnetic girdle" for use with pipelines. U.S. Pat. No. 2,472,851 to Landis utilizes a plurality of coils concentric with an annular electrode to control arc movement in a circular path. U.S. Pat. No. 2,475,183 to Gibson includes a magnet with a pole piece extension to deflect the arc toward the pole piece and thus control its direction during welding. U.S. Pat. No. 3,584,181 to Nemote includes a magnetizing coil that surrounds a rod-shaped workpiece. The magnetizing coil is intended to overcome the circular field created by welding current. Each of these prior art apparatus and methods attempts to control the direction of arc movement by imposing a magnetic field upon it. Such apparatus and methods have been ineffective because of the difficulty in determining and maintaining the proper magnitude and direction of the magnetic field required to achieve the desired effect on the arc. Often the actual effect of such devices is to exacerbate the problem of arc stray.
U.S. Pat. No. 2,994,763 to Schultz and U.S. Pat. No. 3,626,145 to Jackson attempt to overcome the varying magnetic effects by monitoring certain parameters. Schultz uses a pair of photoelectric cells to monitor the position of the arc and an electromagnet that is responsive to the sensed arc position. The electromagnet is intended to produce a force field that compensates for detected drift. Jackson uses a Hall-effect probe mounted immediately ahead of the arc to detect the magnetic environment about the arc. A detector and control circuit is intended to receive signals from the probe and control the current to an electromagnet to set up a "preselected and optimized" magnetic environment. The drawback to such systems is the inability to determine and control an optimum magnetic environment. Systems that attempt to manipulate the arc path with external magnetic fields have been ineffective.
Additional apparatus and methods have been devised for providing a better weld through the use of a controlled magnetic field. Specifically, U.S. Pat. No. 4,107,503 to Koshiya and U.S. Pat. No. 4,716,536 to Blakeley seek to overcome residual or remnant magnetic fields that are found in ferritic materials to be welded. Koshiya is directed to a system to counter magnetic fields created by prior welding operations. In Koshiya, in order to reduce arc blow occurring during welding of the outside of a pipe blank, the direct current in the line feeding the consumable electrode is reversed from that supplied during welding on the inside of the pipe blank. The same magnitude of current is used for welding and for eliminating residual magnetism. This method is often ineffective because the residual magnetism in the pipe blank is not always caused solely by the prior welding operation and simply reversing the current can exacerbate arc blow. Further this system is only applicable to situations where a prior welding operation has been performed and the magnitude and direction of the welding current are known. Blakeley attempts to remove remnant magnetic fields by placing loops of coils on the plates to be welded adjacent to the weld joint. Current is passed through the coils in a direction and magnitude sufficient to counter the remnant magnetic field as measured by a gaussmeter. However, laying of coils in the area of welding can be cumbersome. Further, proper selection of the placement and exact alignment of coils to achieve the desired magnetic effect may be difficult in actual practice.